The field of art to which this invention pertains is hydrocarbon separation. More specifically the invention relates to an improved process for the separation of ethylbenzene from a feed stream comprising ethylbenzene and para-xylene which process employs a solid adsorbent comprising type X or type Y zeolite containing calcium at the exchangeable cationic sites which selectively removes the xylene isomer or isomers from the feed stream thereby producing a raffinate stream comprising ethylbenzene.
2. Description of the Prior Art
Applicant recognizes the abundance of prior art in the separation field especially that art relating to countercurrent fixed bed type operation which are commonly referred to as simulated countercurrent-flow fixed-bed type operations as particularly exemplified in U.S. Pat. No. 2,985,589.
Specific prior art patents which are considered closely related to the present invention are Broughton and Gerhold U.S. Pat. No. 2,985,589; Broughton U.S. Pat. No. 3,274,099; Pharis et al U.S. Pat. No. 3,732,325; Neuzil U.S. Pat. No. 3,696,107; Pharis et al U.S. Pat. No. 3,723,302; Adams et al U.S. Pat. No. 3,733,261; and Broughton U.S. Pat. No. 3,715,409. All of these patents relate to simulated countercurrent solid-fluid separation processes in which an extract component of a feed stream is separated by selective adsorption on a particular adsorbent and subsequently recovered in a higher concentration than that in the feed stream as a product stream. In each process there are various zones representing quantities of adsorbent material in which individual operations are taking place. In each, at least three operational zones are utilized: an adsorption zone, a purification zone and a desorption zone. In the adsorption zone, the selectively adsorbed extract material and perhaps some contaminant materials are adsorbed while the less selectively retained raffinate materials generally remain in the interstitial void spaces surrounding the adsorbent. The basic operation taking place in the purification zone is the purification of the adsorbed extract materials present in the absorbent; the adsorbent in "passing" through the purification zone becomes more concentrated with the extract material and less concentrated with raffinate materials. In the desorption zone a desorbent material removes the adsorbed extract material from the adsorbent.
The first patent discloses the basic concept of a simulated countercurrent solid-fluid contacting process employing a fixed bed of solid adsorbent having moving input and output streams which allow a segregation of zones in which separate functions are taking place in order to separate a feed stream into a raffinate product component and an extract product component.
The second U.S. Pat. No. 3,274,099 includes the same basic processing steps as the first patent but also includes an additional input stream into the purification zone, which is located between the adsorption zone and the desorption zone. The input stream is a sweeping agent, a raffinate-type (that is, a material which is relatively unadsorbed by the adsorbent) compound having a boiling point to permit separation by distillation from the feed raffinate component, which is passed into the process to push raffinate material which is trapped in the interstitial void spaces between adsorbent particles in the purification zone back into an adsorption zone to prevent feed raffinate material from passing from the adsorption zone through the purification zone and into a desorption zone thereby contaminating an extract product with feed raffinate material. In one embodiment, the process of U.S. Pat. No. 3,274,099 is used to separate normal paraffins from isoparaffins.
U.S. Pat. No. 3,732,325 discloses a process which employs the same basic processing steps of the first patent and a particular adsorbent to separate aromatic hydrocarbons, particularly the C.sub.8 aromatics. In the process described in that patent a purification stream which comprises extract material is passed into the purification zone. The extract material can be taken either from an extract outlet stream from the process or from extract material which has been separated from desorbent material in an extract stream fractionator. The purification stream containing the extract material displaces from the interstitial void spaces between the adsorbent particles any raffinate materials carried into the purification zone, removes feed contaminants adsorbed by the adsorbent and reduces the quantity of desorbent which normally surrounds the adsorbent particles in the zone when no purification stream is used.
U.S. Pat. No. 3,696,107 discloses a process for separating para-xylene from a feed stream containing a mixture of C.sub.8 aromatics which employs the basic processing steps described in the first patent, a particular crystalline aluminosilicate adsorbent and a two-stage desorption operation in which a first desorbent stream contacts adsorbent in the desorption zone to effect the desorption of para-xylene from the adsorbent and a second desorbent stream contacts the adsorbent in the desorption zone to effect the pushing of desorbed para-xylenes from the interstitial void spaces between the adsorbent particles. One extract stream is withdrawn from the process.
In U.S. Pat. No. 3,723,302, which discloses a process for separating olefins from paraffins employing the basis processing steps described in the first patent and a particular adsorbent, a two-step desorption operation is again used. The process uses two desorbent materials both of which enter into the desorption zone. The first desorbent material contacts the adsorbent in the desorption zone and causes contaminants to be desorbed from the adsorbent while the second desorbent material is used to desorb the product olefins from the adsorbent contained in the same desorption zone. Two extract streams are withdrawn from the process, an extract contaminant outlet stream and an extract olefin outlet stream.
U.S. Pat. No. 3,733,261 also discloses a process for separating olefins from paraffins which employs the basis processing steps of the first patent mentioned. In that process one desorbent material is admitted in two places in the desorption zone and two extract streams are removed from the process, an extract contaminant stream containing aromatic contaminants and desorbent material and an extract olefin stream containing olefins and desorbent material.
U.S. Pat. No. 3,715,409 discloses a process for the separation of aromatic hydrocarbons which employs four zones and includes the steps of: passing an extract material input stream into the purification zone to effect the desorption and displacement of raffinate material; passing at least a portion of the raffinate output stream passing out of the adsorption zone into the buffer zone to effect desorption and displacement of desorbent material; and, passing a raffinate input stream into an adsorption zone to effect displacement of desorbent from the adsorbent in that zone.
In the process described in my assignee's U.S. Pat. No. 3,917,734 issued to A. J. deRosset, ethylbenzene is recovered in high purity from a feed mixture comprising ethylbenzene and xylene isomers. The process basically comprises contacting the feed mixture with an adsorbent comprising type X or type Y zeolites containing calcium at the exchangeable cationic sites, selectively adsorbing the xylene isomers, and thereafter recovering ethylbenzene as a raffinate component. The adsorbed xylenes may then be recovered, in one embodiment, by contacting the adsorbent with a desorbent material, preferably comprising toluene, thereby desorbing the xylenes and then withdrawing the desorbed xylenes from the adsorbent. In a preferred embodiment the adsorption and desorption are done continuously in a simulated moving-bed countercurrent-flow system, the operating principles and sequence of which are described in U.S. Pat. No. 2,985,589.
It has been discovered that when the feed mixture to this deRosset proesss includes para-xylene and when the preferred toluene desorbent material is employed the selectivity of that adsorbent is higher for the toluene desorbent material than it is for para-xylene. This results in the inability of that process to obtain high purity product and high yields simultaneously when the para-xylene concentration of the feed is about the same as or less than that of ethylbenzene. The process of my invention eliminates that problem thereby making separation of ethylbenzene from xylene isomers in both high purity (98% or greater, expressed as a percent of C.sub.8 aromatics present) and high yields (95% or greater) possible for any ethylbenzene concentration in the feed.
Ethylbenzene, used as a raw material in the production of styrene monomer, is commercially produced from the alkylation of benzene with ethylene. The cost of and competing demands for necessary benzene and ethylene feed streams have, however, prompted new efforts to recover ethylbenzene from various C.sub.8 aromatic feed streams which already contain ethylbenzene. Such feed streams for instance include C.sub.8 aromatic extracts produced by a typical solvent extraction process for a pyrolysis gasoline or from a naphtha which has been reformed with a platinum-halogen-containing catalyst. Additionally, C.sub.8 aromatic cuts of hydrogenated pyrolysis naphthas or reformates prepared by fractionation without solvent extraction contain varying amounts of ethylbenzene. The particular utility of the process of my invention therefore is that it offers a method for recovering ethylbenzene from a feed stream which already contains ethylbenzene.
Ethylbenzene can, of course, be separated from the xylene isomers by fractionation but because its boiling point is within about 4.degree. F. of that of para-xylene, the fractionation can be achieved only with the more intricate super-fractionators. Typical ethylbenzene fractionators contain 300 to 400 actual trays and require about a 25--50 to 1 reflux to feed ratio. The process of my invention therefore offers a competitive alternative to the separation of ethylbenzene by superfractionation.